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Best way to weld SS to prevent crevice corrosion??

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Best way to weld SS to prevent crevice corrosion??

Postby lostsoul » Tue May 09, 2017 10:47 pm

I am rebuilding a sailboat rudder.
It has an internal ss framework of a 1 1/2 inch pipe with three 1/4 X 1 1/2 inch flat bars welded to it to make ribs across the inside of the rudder. I will post pictures, if I can.

I need to have the ribs welded to the post. I just bought a 4 foot length of 316 ss 1 1/2 pipe and 4 foot of 1/4 by 1 1/2 flat bar (316L) from online metals.

The ribs will be about 18 long and will be attached to the post at 90 degrees. The whole thing fits inside a fiberglass rudder and is submerged in sea water. Surrounding the ss frame is poured expanding foam. This gives strength and helps keep sea water out. But inevitably seawater will get in. The ss will be at risk of crevice corrosion due to the seawater and no oxygen.

The rudder details are not important. What I have is a ss weld that will be exposed to seawater in a no oxygen environment.

I am looking for advice on how one would weld this to prevent crevice corrosion. TIG? MIG? what rod? Pickling? Passivization?

I am only a basic welder so will bring it to a welder. Problem is that the welders i have been to really dont seem to understand my needs and dont know what crevice corrosion is. I am still looking for a better welder. But in the mean time, maybe some one can tell what the best way to approach the welds would be.

The picture shows the rudder frame. You can see the ribs with one broken off. The original frame was made with a ss post and plain steel ribs. You can see one broken off. Only about half of the post is encased in the fiberglass rudder. The top half, with no ribs, extends into the boat to the steering gear.
Next to the rudder frame is the new post and the rib material.

I am frustrated because I am researching this stuff and it seems pretty common, yet the welders dont know. What do I do?
I am in SW Florida in the Tampa/St Pete-Sarasota-Ft Myers area, in case someone has a shop to recommend.

Thank you!!!
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Re: Best way to weld SS to prevent crevice corrosion??

Postby Poland308 » Tue May 09, 2017 11:47 pm

Match your materials and your filler rod. I.E. all 316 ss if that's what you've got. Dissimilar metals in a salt brine solution is a prime recipe for galvanic corrosion. Then you should be fine. Notice how the ss tube on your old part is only corroded near the areas where the two different metals were joined. Tig weld it. If your really worried about corrosion and money isn't an object then go with some hasteloy or inconell, but even then you need to keep the materials matched with the filler.
I have more questions than answers

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Re: Best way to weld SS to prevent crevice corrosion??

Postby cj737 » Wed May 10, 2017 8:52 am

Irrespective of the filler rod, this will not prevent crevice corrosion. It occurs (in this example) because seawater becomes trapped and stagnant within the rudder.

I'll freely admit, I have never contemplated this condition, but (thinking aloud now...) I wonder if coating the stainless in powder coat then installing it within the expanded foam would protect it? Or some other type of polymer coating? I will agree with Poland, you also need to insure the filler wire is same-or-higher grade stainless to your parent metals, but that's a separate issue.
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Re: Best way to weld SS to prevent crevice corrosion??

Postby exnailpounder » Wed May 10, 2017 2:01 pm

Poland308 wrote:Match your materials and your filler rod. I.E. all 316 ss if that's what you've got. Dissimilar metals in a salt brine solution is a prime recipe for galvanic corrosion. Then you should be fine. Notice how the ss tube on your old part is only corroded near the areas where the two different metals were joined. Tig weld it. If your really worried about corrosion and money isn't an object then go with some hasteloy or inconell, but even then you need to keep the materials matched with the filler.

I have to agree with this^^^. Any idea of how long that rudder was in service before you took it apart and found the damage? The SS post looks to have come through ok. I am thinking that if you tig it and repassivate it, it will last a very long time. Any ideas about what others are doing about this problem?
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Re: Best way to weld SS to prevent crevice corrosion??

Postby LtBadd » Wed May 10, 2017 4:13 pm

exnailpounder wrote:Any ideas about what others are doing about this problem?


Making it from Titanium? ;) That would last awhile
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Re: Best way to weld SS to prevent crevice corrosion??

Postby exnailpounder » Wed May 10, 2017 4:25 pm

LtBadd wrote:
exnailpounder wrote:Any ideas about what others are doing about this problem?


Making it from Titanium? ;) That would last awhile

It wouldn't be that expensive to do it in Ti but whose gonna weld it? You're not that far from the OP ;)
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Re: Best way to weld SS to prevent crevice corrosion??

Postby LtBadd » Wed May 10, 2017 6:25 pm

exnailpounder wrote:
LtBadd wrote:
exnailpounder wrote:Any ideas about what others are doing about this problem?


Making it from Titanium? ;) That would last awhile

It wouldn't be that expensive to do it in Ti but whose gonna weld it? You're not that far from the OP ;)

I have reached out to see if I can help, will talk to him soon. I did recently buy the Furick #12 pyrex cup, be a good way to break that in. :P
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Re: Best way to weld SS to prevent crevice corrosion??

Postby Otto Nobedder » Wed May 10, 2017 6:43 pm

I would use 316 throughout, and an over-alloyed (fully austinitic) filler. Eliminating ferrite in the weld will go a long way to stopping crevice corrosion. Here's a quote from a discussion I had recently with a welding engineer, "Examples of filler metals which solidify austenitically are AXT, 310, 310L, 383, 385, and a urea grade 25.22.2.LMn."

Note, the reason most common fillers leave residual ferrite is to prevent hot-cracking of the weld, which means you have to control both heat input (Pre-heat is advised) and cooling rate, perhaps even peening the weld during it's initial cool to interpass temperature.

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Re: Best way to weld SS to prevent crevice corrosion??

Postby exnailpounder » Wed May 10, 2017 7:10 pm

Otto Nobedder wrote:I would use 316 throughout, and an over-alloyed (fully austinitic) filler. Eliminating ferrite in the weld will go a long way to stopping crevice corrosion. Here's a quote from a discussion I had recently with a welding engineer, "Examples of filler metals which solidify austenitically are AXT, 310, 310L, 383, 385, and a urea grade 25.22.2.LMn."

Note, the reason most common fillers leave residual ferrite is to prevent hot-cracking of the weld, which means you have to control both heat input (Pre-heat is advised) and cooling rate, perhaps even peening the weld during it's initial cool to interpass temperature.

Steve

So..in your opinion..would 316 work throughout? I think it would. I have been in refineries and seen all types of SS take some serious abuse and hang in there. Refineries are not the place to say...maybe this'll work :lol:
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Re: Best way to weld SS to prevent crevice corrosion??

Postby lostsoul » Wed May 10, 2017 8:02 pm

The rudder is a 1974 on a 1974 boat. I have owned the boat for 20 years.
As far as I know, the rudder was not rebuilt before I owned it, but I cant be sure. There are many other boats of the same model and year and some owners have rebuilt theirs. Many just scrape out the old foam and dont really expose the welds. But others have done something similar.

But this method of rudder building is common. It tends to last a long time. I could buy a replacement rudder which would be built in the same manner. Big bucks though. There is a place called Foss Foam that makes new rudders. I think they use all SS and not a more corrosion resistant alloy. Thing is to keep the sea water out is key. But it will get in at some point, it is just a matter of time.

I have the 316 post and ribs pretty much set to be welded. I stopped by a shop and they said "no problem, be less then $100". But they dont seem to understand that this welding needs to be well done to be corrosion resistant. For me, I dont know exactly what they should do. Maybe I should trust them? I would like to learn more about this so I get it dont the best it can be. After all I dont want to have my rudder fail. It is not like a flat tire.

I know that the old ss post was welded to plain steel for 40 years and did not fail. (almost). I just like to do things the right or best way. I am not a good enough kind of guy.


Thanks for the comments guys!!
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